CAIRO — Hundreds of thousands thronged the streets of Cairo and cities across the country Sunday and marched on the presidential palace, filling a broad avenue for blocks, in an attempt to force out the Islamist president with the most massive protests Egypt has seen in 2½ years of turmoil.

A hard core of young opponents broke away from the rallies and attacked the main headquarters of President Mohammed Morsi’s Muslim Brotherhood, pelting it with stones and firebombs until a raging fire erupted in the walled villa. During clashes, Brotherhood supporters opened fire on the attackers, and activists said three protesters were killed.

Fears were widespread that the two sides could be heading to a violent collision in coming days. Morsi made clear through a spokesman that he would not step down and his Islamist supporters vowed not to allow protesters to remove one of their own, brought to office in a legitimate vote. Thousands of Islamists massed not far from the presidential palace in support of Morsi, some prepared for a fight with makeshift armor and sticks.

At least five anti-Morsi protesters were killed Sunday in clashes and shootings in southern Egypt.

The protesters aimed to show by sheer numbers that the country has irrevocably turned against Morsi, a year to the day after he was inaugurated as Egypt’s first freely elected president. But throughout the day and even up to midnight at the main rallying sites, fears of rampant violence did not materialize.

Instead, the mood was largely festive as protesters at giant anti-Morsi rallies in Cairo’s central Tahrir Square and outside the Ittihadiya palace spilled into side streets and across boulevards, waving flags, blowing whistles and chanting.

“Mubarak took only 18 days although he had behind him the security, intelligence and a large sector of Egyptians,” said Amr Tawfeeq, an oil company employee marching toward Ittihadiya with a Christian friend. Morsi “won’t take long. We want him out, and we are ready to pay the price.”

The outpouring against Morsi raises the question of what is next. Protesters have vowed to stay on the streets until he steps down, and organizers called for widespread labor strikes starting Monday. The president, in turn, appears to be hoping protests wane.

In Cairo and a string of cities in the Nile Delta and on the Mediterranean coast, the protests topped even the biggest protests of the 2011’s 18-day uprising, including the day Mubarak quit, Feb. 11, when giant crowds marched on Ittihadiya.

It is unclear now whether the opposition, which for months has demanded Morsi form a national unity government, would now accept any concessions short of his removal. The anticipated deadlock raises the question of whether the army, already deployed on the outskirts of cities, will intervene.

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